Modulation doped tunnel junction

ABSTRACT

A vertical cavity surface emitting laser (VCSEL) includes a substrate; a first mirror stack over the substrate; an active region having a plurality of quantum wells over the first mirror stack; a tunnel junction over the active region, the tunnel junction including a modulation-doped layer; and a second mirror stack over the tunnel junction. The modulation doped layer can be used for either the n-layer or the p-layer, or the both layers of the tunnel junction. Such tunnel junctions are especially useful for a long wavelength VCSEL.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to vertical cavity surface emitting lasers(VCSELs). More specifically, it relates to tunnel junctions for VCSELs.

2. Discussion of the Related Art

Vertical cavity surface emitting lasers (VCSELs) represent a relativelynew class of semiconductor lasers. While there are many variations ofVCSELs, one common characteristic is that they emit light perpendicularto a wafer's surface. Advantageously, VCSELs can be formed from a widerange of material systems to produce specific characteristics.

VCSELs include semiconductor active regions, which can be fabricatedfrom a wide range of material systems, distributed Bragg reflector (DBR)mirrors, current confinement structures, substrates, and contacts. SomeVCSELs, particularly those used at long-wavelengths, incorporate tunneljunctions. Because of their complicated structure, and because of theirmaterial requirements, VCSELs are usually grown using metal-organicchemical vapor deposition (MOCVD).

FIG. 1 illustrates a typical long-wavelength VCSEL 10 having a tunneljunction. As shown, an n-doped InP substrate 12 has an n-type electricalcontact 14. An n-doped lower mirror stack 16 (a DBR) is on the InPsubstrate 12, and an n-type graded-index InP lower spacer 18 is disposedover the lower mirror stack 16. An InGaAsP or AlInGaAs active region 20,usually having a number of quantum wells, is formed over the InP lowerspacer 18. Over the active region 20 is a tunnel junction 28. Over thetunnel junction 28 is an n-type graded-index InP top spacer 22 and ann-type InP top mirror stack 24 (another DBR), which is disposed over theInP top spacer 22. Over the top mirror stack 24 is an n-type conductionlayer 9, an n-type cap layer 8, and an n-type electrical contact 26.

Still referring to FIG. 1, the lower spacer 18 and the top spacer 22separate the lower mirror stack 16 from the top mirror stack 24 suchthat an optical cavity is formed. As the optical cavity is resonant atspecific wavelengths, the mirror separation is controlled to resonate ata predetermined wavelength (or at a multiple thereof). At least part ofthe top mirror stack 24 includes an insulating region 40 that providescurrent confinement. The insulating region 40 is usually formed eitherby implanting protons into the top mirror stack 24 or by forming anoxide layer. In any event, the insulating region 40 defines a conductiveannular central opening 42 that forms an electrically conductive paththough the insulating region 40. When such a current confinementstructure is adopted for a VCSEL, the tunnel junction 28 can also beplaced above the current confinement structure (here, the insulatingregion 40).,

In operation, an external bias causes an electrical current 21 to flowfrom the electrical contact 26 toward the electrical contact 14. Theinsulating region 40 and the conductive central opening 42 confine thecurrent 21 such that the current flows through the conductive centralopening 42 and into the tunnel junction 28. The tunnel junction convertsincoming electrons into holes that are injected into the active region20. Some of the injected holes are converted into photons in the activeregion 20. Those photons bounce back and forth (resonate) between thelower mirror stack 16 and the top mirror stack 24. While the lowermirror stack 16 and the top mirror stack 24 are very good reflectors,some of the photons leak out as light 23 that travels along an opticalpath. Still referring to FIG. 1, the light 23 passes through theconduction layer 9, the cap layer 8, an aperture 30 in electricalcontact 26, and out of the surface of the vertical cavity surfaceemitting laser 10.

It should be understood that FIG. 1 illustrates a typicallong-wavelength VCSEL having a tunnel junction, and that numerousvariations are possible. For example, the dopings can be changed (say,by providing a p-type substrate), different material systems can beused, operational details can be tuned for maximum performance, andadditional structures and features can be added.

While generally successful, VCSELs similar to that illustrated in FIG. 1have problems. One problem in realizing commercial quality longwavelength VCSELs, which is addressed by the tunnel junction 28, isoptical loss. In long wavelength VCSELs, it is often critical to limitoptical losses. To that end, p-doped materials, which absorb more lightthan n-doped materials, are replaced by n-doped materials and the tunneljunction 28. That junction converts electron currents into hole currentsthat are injected into the active region. That way, long wavelengthVCSELs can be made with a non-absorbing n-type mirror on both the topand the bottom.

Tunnel junctions used in semiconductor lasers are thin (say 10nanometer), reversed biased structures. Such tunnel junctions areusually n++/p++ structures in which the both sides have a high dopingdensity using usually a low diffusivity dopant. This enables a lowvoltage drop across the tunnel junctions, and low free carrierabsorption and sufficient free carriers in the semiconductor lasers.Dopants for such tunnel junctions should provide a high doping level insemiconductor matrix and should not diffuse around so as to maintain asharp doping profile. Because of their low diffusivity, silicon andcarbon are used for n-type and p-type dopants in tunnel junctions,respectively, especially for non-indium containing GaAs based VCSELs.For Indium-Alloy-based VCSELs, which are more pertinent tolong-wavelength VCSELs, however, high doping density for p-type usingcarbon is, however, difficult to obtain. For example, p-type dopingdensity of AlInAs lattice matched to InP with conventional dopingtechniques is less than 5×10¹⁹ cm⁻³, which is an amount of dopingdensity for a good tunnel junction with low voltage drop. Similarly,high doping density for n-type using silicon is also difficult toobtain.

Thus, a new long wavelength VCSEL would be beneficial. Even morebeneficial would be a new tunnel junction suitable for use in longwavelength VCSELs. Still more beneficial would be a new tunnel junction,in which both n-layer and p-layer are heavily doped (higher than 5×10¹⁹cm⁻³) such that voltage drop across the tunnel junction is small enoughto design a VCSEL with improved performance.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a tunnel junctionsuitable for use in long wavelength VCSELs that substantially obviatesone or more of the problems due to limitations and disadvantages of theprior art.

A principle of the present invention is to provide a tunnel junctionthat utilizes a modulation doped layer for either the n-layer or thep-layer, or both layers of the tunnel junction. A tunnel junctionaccording to the principles of the present invention includes such amodulation-doped layer (either n-layer or p-layer). Beneficially, such atunnel junction is formed above quantum wells to form a VCSEL. SuchVCSELs are particularly advantageous at long wavelengths.

A VCSEL having a tunnel junction constructed according to the principlesof the present invention has either a high p-doping density (forexample, higher than 5×10¹⁹ cm⁻³) for the p-side of the tunnel junctionor a high n-doping density (for example, higher than 5×10¹⁹ cm⁻³) forthe n-side of the tunnel junction, or both. Thus, the tunnel junctionconstructed according to the principles of the present invention has anenhanced conductive tunneling characteristic, which in turn results inan improved performance of the VCSEL.

In order to achieve these and other advantages and in accordance withthe purpose of the present invention, as embodied and broadly described,a vertical cavity surface emitting laser may, for example, include asubstrate; a first mirror stack over the substrate; an active regionhaving a plurality of quantum wells over the first mirror stack; atunnel junction over the active region, the tunnel junction including amodulation doped layer; and a second mirror stack over the tunneljunction.

In another aspect of the present invention, a tunnel junction may, forexample, include a modulation-doped layer.

In yet another aspect of the present invention, a long wavelength VCSELmay, for example, include an indium-based semiconductor substrate; afirst mirror stack over the substrate; an active region having aplurality of quantum wells over the first mirror stack; a tunneljunction over the active region, the tunnel junction including amodulation-doped layer; and a second mirror stack over the tunneljunction.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thatdescription, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and which are incorporated in andconstitute a part of this specification, illustrate embodiments of theinvention and together with the description serve to explain theprinciples of the invention.

In the drawings:

FIG. 1 illustrates a typical long-wavelength vertical cavity surfaceemitting laser;

FIG. 2 illustrates a first method of a modulation doping according to aprinciple of the present invention in which a digital alloy of p-dopedAlAs and InAs produce AlInAs layer with a high doping level;

FIG. 3 illustrates a second method of a modulation doping according to aprinciple of the present invention in which very thin layers of SiAs ineither n-type InGaAs or non-intentionally doped InGaAs produce InGaAswith a high doping level;

FIG. 4 illustrates a vertical cavity surface emitting laser that is inaccord with the principles of the present invention;

FIG. 5 illustrates a tunnel junction structure in the VCSEL illustratedin FIG. 4 that is in accord with the principles of the presentinvention; and

FIG. 6 illustrates a tunnel junction structure in the VCSEL illustratedin FIG. 4 that is in accord with the principles of the present inventionin which either the p-layer and the n-layer of the tunnel junction isformed by a modulation doped layer according to the principles of thepresent invention.

Note that in the drawings that like numbers designate like elements.Additionally, for explanatory convenience the descriptions usedirectional signals such as up and down, top and bottom, and lower andupper. Such signals, which are derived from the relative positions ofthe elements illustrated in the drawings, are meant to aid theunderstanding of the present invention, not to limit it.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

A principle of the present invention is to provide a tunnel junctionthat utilizes a modulation doping method for either the n-layer or thep-layer, or the both layers of the tunnel junction. The modulationdoping methods utilizes the principle that carriers (electrons or holes)electrically behave in two semiconductor layers formed directlycontacting each other, as if the carriers are in a single-compoundsemiconductor matrix having the average composition and doping densityof the two semiconductor layers, so long as the total thickness of thetwo semiconductor layers is in a range of 0.1 nm˜10 nm, with optimumthickness being in a range of 1 nm˜2 nm. In one aspect of the presentinvention the tunnel junction has an effective carrier concentrationgreater than about 1×10¹⁹ cm⁻³.

The modulation doping according to the present invention can beaccomplished by two basic methods. The first method includes forming atleast two layers alternatively, the two layers together constituting acompound semiconductor matrix in terms of its composition and dopingdensity and at least one layer capable of being highly doped with adesired dopant using the conventional doping methods in the VCSELtechnology, such as Metal Organic Chemical Vapor Deposition (“MOCVD”)and Molecular Beam Epitaxy (“MBE”) methods. In another aspect of thepresent invention, at least one of the layers may, itself, comprise adopant layer. For example, AlInAs 70 can be epitaxially grown as adigital alloy of AlAs 75 and InAs 72, as illustrated in FIG. 2. BecauseAlAs 75 can be p-type doped 2×10²⁰ cm⁻³ or more (e.g., 5×10¹⁹ cm⁻³ orgreater), the digital alloy of AlAs and non-intentionally doped InAs canproduce AlInAs 70 of effective doping level of 1×10²⁰ cm⁻³, which isuseful for making a good tunnel junction with low voltage drop acrossthe tunnel junction. In this example, the doping is usually performedusing carbon as a dopant. The total thickness of a pair of InAs and AlAsis in a range of 0.1 nm˜10 nm, with optimum thickness being in a rangeof 1 nm˜2 nm. N-type doping can be also achieved using this firstmethod.

The second basic method is inserting doping layers without digitallyalloying the semiconductor itself, which is called “delta doping”. Thismethod is especially useful for silicon doping of many III–Vsemiconductors. For example, the doping level of silicon in InGaAssaturates at approximately 2×10¹⁹ cm⁻³. However, interlacing very thinlayers of SiAs 95 in either n-type InGaAs or non-intentionally dopedInGaAs 92 can produce doping levels well exceeding 1×10¹⁹ cm⁻³ (e.g.,5×10¹⁹ cm⁻³ or greater), as illustrated in FIG. 3. As with the firstmethod, the total thickness of a pair of n-type InGaAs ornon-intentionally-doped InGaAs 92 and SiAs 95 is in a range of 0.1 nm˜10nm, with optimum thickness being in a range of 1 nm˜2 nm. In the secondmethod, the thickness of SiAs 95 is beneficially, however, 1/1000 of thethickness of n-type InGaAs or non-intentionally doped InGaAs 92. A highdoped AlGaInAs can also be obtained using this method by interlacingvery thin layers of SiAs into AlGaInAs. P-type doping can also beachieved using this second method. MOCVD and MBE methods are generallyused to grow SiAs layer 95, and silicon is used for n-type dopants inthis example.

The principles of the present invention are now incorporated in anembodiment including a VCSEL having a bottom InGaAsP/InP DBR mirrorgrown on an InP substrate. An example of such a VCSEL is the VCSEL 100illustrated in FIG. 4.

As shown in FIG. 4, the VCSEL 100 includes an n-doped InP substrate 112having an n-type electrical contact (not shown for clarity). Over theInP substrate 112 is an n-doped lower mirror stack 116 (a DBR) comprisedof a plurality of alternating layers of AlGaInAs/AlInAs, InAlGaAs/InP orInGaAsP/InP. Over the lower mirror stack 116 is an n-doped InP spacer118. The lower mirror stack 116 is beneficially grown on the InPsubstrate using, for example, common metal-organic and hydride sourceslike TMAl, TMGa, PH₃ and AsH₃ in an MOCVD process. Then, the InP spacer118 is grown, also using MOCVD. An active region 120 comprised of P-Njunction structures and having a number of quantum wells is then formedover the InP spacer 118. The composition of the active region 120 isbeneficially InGaAsP or AlInGaAs. Over the active region 120 is a p-typeInP top spacer 121. Similar to the lower InP spacer 118, the p-type InPtop spacer 121 is also grown using MOCVD.

Referring to FIG. 4 and FIG. 5, over the p-type InP top spacer 121 is atunnel junction 122 comprised of a reverse biased n++/p++ junction. ForVCSELs, the tunnel junction 122 generally includes a p-layer 210 and ann-layer 220, with both materials selected from, for example, InP,AlInAs, AlInGaAs, InGaAsP, GaAs, AlAs, AlGaAs, InGaAs, AlGaAsSb, GaAsSb,AlAsSb, AlPSb, GaPSb, AlGaPSb, and combinations thereof. According tothe principles of, the present invention, several embodiments arepossible to form the tunnel junction 122 by replacing either the p-layer210 or the n-layer 220, or by replacing the both layers 210 and 220 witha modulation doped layer that is in accord with a principle of thepresent invention. In one embodiment for the tunnel junction 122, theAlInAs 70 in FIG. 2, which is a combination of the digital alloy ofp-doped AlAs 75 and the InAs layer 72, replaces the p-layer 210. Inanother embodiment, the InGaAs 90 having the thin layer of SiAs 95 inFIG. 3 replaces the n-layer 220. In yet another embodiment, the tunneljunction 122 is formed by using the AlInAs 70 for the p-layer 210 and byusing the InGaAs 90 having the thin layer of SiAs 95 for the n-layer220, as illustrated in FIG. 6.

MOCVD and MBE methods are utilized to grow all the layers in theseembodiments of the tunnel junction 122. For the MOCVD method, TMAM,TMGa, TMSb and PH₃ sources are used to grow the layers in the tunneljunction 122 in a growth temperature between 400° C. and 900° C. Thegrowth rate and the composition can be easily controlled by adjustinggroup III and V source inputs. P-type doping is beneficially performedusing carbon sources, and n-type doping is beneficially performed usingsilicon sources. When the MBE method is used to grow the layers in thetunnel junction, the Equivalent Beam Pressures (“EBP”) of group Vsources, such as Sb, P and As, are in a range of about 1×10⁻⁷˜about1×10⁻³ torr, with the optimum EBP being ˜1×10⁻⁵ torr, and the growthrates of group III sources, such as In, Al and Ga, are less than 10μm/hour, with the optimum growth rate being ˜1 μm/hour. When utilizingthe second method of the modulation doping using MBE, the growth rate ofSiAs should be controlled such that the thickness of SiAs 95 isbeneficially ˜ 1/1000 of the thickness of n-type InGaAs ornon-intentionally doped InGaAs 92.

Over the tunnel junction 122 is an n-type top mirror structure (whichincludes another DBR). As in the case of the lower mirror stack 116, then-type top mirror stack 132 is beneficially comprised of a plurality ofalternating layers of AlGaInAs/AlInAs, InAlGaAs/InP or InGaAsP/InP, andis beneficially grown using MOCVD. As shown in FIG. 2, an insulatingstructure 130 may be optionally provided between the tunnel junction 122and the n-type top mirror stack 132 (or between the tunnel junction 122and the active region 120). The insulating structure includes an opening131, which enables current flow through the VCSEL 100. The result is ahigh-quality mirror stack 132 having current confinement.

With the mirror stack 132 formed, an n-type conduction layer (similar tothe p-type conduction layer 9 of FIG. 1), an n-type GaAs cap layer(similar to the p-type GaAs cap layer 8 of FIG. 1), and an n-typeelectrical contact (similar to the p-type electrical contact 26 ofFIG. 1) may be provided to complete the VCSEL 100.

The VCSEL 100 having a tunnel junction 122 constructed according to theprinciples of the present invention has significant advantages overprior art VCSELs. First, since a high doping density (higher than 5×10¹⁹cm⁻³) is obtainable for either the p-layer or the n-layer, or the bothlayers of the tunnel junction, there is a lower voltage drop across thetunnel junction portion 122 in the VCSEL 100 as compared with theconventional VCSELs. The overall result is a VCSEL with improvedperformance.

It should be understood that, while a tunnel junction according to theprinciples of the present invention is described in this section with along wavelength VCSEL device, it could be used with other types of VCSELor other devices for its intended purposes herein described. It shouldbe further understood that, while a tunnel junction is provided over theactive region 120 in this section, the number and/or location of thetunnel junction could be varied in VCSELs.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A vertical cavity surface emitting laser comprising: a substrate; afirst mirror stack over the substrate; an active region having aplurality of quantum wells over the first mirror stack; a tunneljunction over the active region, the tunnel junction including: ap-layer that includes a modulation doped layer comprising an AlInAslayer epitaxially grown as a digital alloy of p-type doped AlAs andInAs; and an n-layer that includes an n-layer of a compound selectedfrom the group consisting of InP, AlInAs, AlInGaAs, InGaAsP, GaAs, AlAs,AlGaAs, InGaAs, AlGaAsSb, GaAsSb, AlAsSb, AlPSb, GaPSb, AlGaPSb, andcombinations thereof; and a second mirror stack over the tunneljunction.
 2. A vertical cavity surface emitting laser according to claim1, wherein the modulation layer is doped with a concentration greaterthan 1×10¹⁹ cm⁻³.
 3. A vertical cavity surface emitting laser accordingto claim 1, wherein the p-type AlAs layer is doped with carbon to aconcentration greater than 1×10¹⁹ cm⁻³, and wherein an effective dopingconcentration of the modulation doped layer is greater than 1×10¹⁹ cm⁻³.4. A vertical cavity surface emitting laser according to claim 1,further including an n-type spacer adjacent the active region, andwherein the first mirror stack is an n-type DBR.
 5. A vertical cavitysurface emitting laser according to claim 1, further including a p-typespacer adjacent the tunnel junction, and wherein the second mirror stackis an n-type DBR.
 6. A vertical cavity surface emitting laser accordingto claim 1, further including: an n-type bottom spacer adjacent theactive region, and wherein the first mirror stack is an n-type DBR; anda p-type top spacer adjacent the tunnel junction, wherein the first andsecond mirror stacks are each an n-type DBR.
 7. A vertical cavitysurface emitting laser according to claim 1, wherein the p-layer isdoped with carbon with a concentration greater than 1×10¹⁹ cm⁻³.
 8. Avertical cavity surface emitting laser according to claim 1, wherein theactive region includes one of InGaAsP and AlInGaAs.
 9. A vertical cavitysurface emitting laser according to claim 1, wherein the first andsecond mirror stacks are lower and upper mirror stacks, respectively.10. A vertical cavity surface emitting laser comprising: a substrate; afirst mirror stack over the substrate; an active region having aplurality of quantum wells over the first mirror stack; a tunneljunction over the active region, the tunnel junction including: ann-layer that includes a modulation doped layer comprising an SiAs layerand an AlGaInAs layer; and a p-layer that includes a p-layer of acompound selected from the group consisting of InP, AlInAs, AlInGaAs,InGaAsP, GaAs, AlAs, AlGaAs, InGaAs, AlGaAsSb, GaAsSb, AlAsSb, AlPSb,GaPSb, AlGaPSb, and combinations thereof; and a second mirror stack overthe tunnel junction.
 11. A vertical cavity surface emitting laseraccording to claim 10, wherein the thickness of the SiAs layer is about1/1000 of the AlGaInAs layer.
 12. A vertical cavity surface emittinglaser according to claim 10, wherein an effective doping concentrationof the modulation doped layer is greater than 1×10¹⁹ cm⁻³.
 13. A tunneljunction including a modulation doped layer that comprises an AlInAslayer epitaxially grown as a digital alloy of p-type doped AlAs andInAs.
 14. A tunnel junction according to claim 13, wherein themodulation-doped layer is doped with an effective carrier concentrationgreater than 1×10¹⁹ cm⁻³.
 15. A tunnel junction according to claim 13,wherein a total thickness of the modulation-doped layer is in a range ofabout 0.1 nm˜about 2 nm.
 16. A tunnel junction according to claim 13,wherein the p-type AlAs layer is doped with carbon to a concentrationgreater than 1×10¹⁹ cm⁻³, and wherein an effective doping concentrationof the modulation doped layer is greater than 1×10¹⁹ cm⁻³.
 17. A tunneljunction according to claim 13, wherein the tunnel junction includes ann-type layer.